Plasma deposition is a coating method wherein thin films are deposited from a gas, aerosol, liquid or vapor state to a solid state on a substrate, and may be used for coating virtually any substrate with a vast choice of coatings. Unlike other coating techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) which are limited to batch processes, plasma deposition is suitable for use in continuous coating processes, and therefore highly attractive for industrial applications.
However, as the coatings obtained by plasma deposition often only have a thickness of a few nanometers, the in-line characterization of such coatings in an industrial process remains a challenge. Yet, the detection of small deviations of the coating quality is of high importance for timely adjustment of the coating process, such that larger production errors can be avoided.
Currently, various techniques are available for coating characterization in general, including such techniques as x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive x-ray analysis (EDX), profilometry, contact angle measurement, Fourier transform infrared spectroscopy (FTIR), broadband optical monitoring (BOM), and atomic force microscopy (AFM). Although these techniques may allow the accurate determination of one or more aspects of the coating such as thickness, chemical composition, hydrophilicity or hydrophobicity, and roughness, they are not suitable for the in-line monitoring of continuous coating processes.
Accordingly, there is a need for improved processes which allow the characterization of coatings obtained by plasma deposition.
Patent application GB2455849 describes plasma spray deposition of barrier coatings comprising rare earth elements as taggants. Using a high temperature plasma gas stream, high melting point materials such as metals are sprayed onto a work piece. EP1643005 describes vacuum plasma deposition which is a batch process. Dye molecules are used to for applications in opto- and microelectronics.